A gas turbine usually comprises a compressor, a combustor and a turbine. The compressor compresses air which is fed to the combustor where it is mixed with fuel. Inside a combustion chamber the resulting fuel-air mixture is combusted. During the combustion of fuel and air hot combustion gas is generated. This combustion gas is used to drive the turbine. A typical combustor comprises a burner, a pre-chamber which is located close to the burner, and a main combustion chamber. Especially the pre-chamber is exposed to very high temperatures due to its location near the burner. If the pre-chamber reaches a certain temperature it is prone to carbon build-up and then the metal of the pre-chamber casing may be negatively affected.
To protect the pre-chamber components cooling holes are added to the pre-chamber casing. Cooling with, for instance, compressed air limits the maximum temperature of the pre-chamber. However, the air which is used for cooling could also otherwise be doing work in the turbine and thus has an impact on the efficiency of the turbine, even though it is only a minor impact.